The present invention relates to devices for conveying and guiding lead-in strips of webs in paper machines which utilize means for producing vacuum effects to cause the lead-in strips to adhere to and be kept in contact with runs of conveyor belts of the paper machines.
As known in prior art, when a paper machine is started or after a web break, a tail of the web is passed through the paper machine by cutting from the web a narrow lead-in strip, which is guided manually through the machine by using air jets as well as different guide plates and threading devices. Continuously increasing running speeds of paper machines have caused increasing problems in threading of the web and thus new types of arrangements have been needed in order to accomplish threading of the web.
With respect to the prior art relating to the invention, reference is made to U.S. Pat. No. 3,355,349, which discloses a belt conveyor intended for transfer of a lead-in strip to a calender or to a reel-up, or a belt conveyor disposed before a calender. This known belt conveyor comprises two reversing rolls and a closed and air pervious belt loop disposed therebetween and having an upper run which is subjected to a vacuum. This vacuum is produced by means of a suction box which is placed inside the belt loop and which creates a vacuum effect on the upper run of the belt to keep the lead-in strip in contact with the conveyor belt. A drawback in this known device has been that the device which is based on a suction box is rather complex and heavy in structure and it includes a large number of wearing parts and takes much space. This known device lacks the possibility of profiling in a longitudinal direction, and in terms of servicing it is not advantageous. In this arrangement known from prior art, there is a high vacuum on the entire run with the result that there is created heavy friction, and thus large motors are required for conveying the conveyor belt and the web. The purpose of the present invention is to develop further the above-mentioned conveyor device so that the above-noted drawbacks may be avoided.
With respect to the prior art relating to the invention, reference is also made to FI Patent 69145, which discloses a device for conveying and guiding a lead-in strip of a web in a paper machine. This prior-art device comprises a conveyor belt arranged around two or more reversing rolls, which belt is pervious to air and has devices arranged within its loop for producing a vacuum effect on the run of the conveying belt on which the lead-in strip is conveyed, the lead-in strip being caused to adhere to and be held in contact with the run of the conveyor belt by means of the vacuum effect. On the conveying run of the conveyor belt, inside its loop, there are provided air blow means which include guide plates extending substantially parallel to the plane of the conveyor belt and the conveying run, in connection with which plates a dynamic vacuum effect can be produced by means of air blowings, the lead-in strip being caused to adhere to and be kept in contact with the conveying run of the conveyor belt by means of the vacuum effect. This known arrangement requires an external source of air and a rather large amount of air. This known device suffers from the problem that the air blow means placed one after the other in the running direction of the belt produce a wavelike vacuum curve, which changes from a negative pressure into a positive pressure just before the next air blow means. A problem in this kind of device is that it may cause the web to form bights at the areas with a positive pressure. The purpose of the invention is to develop further this known conveyor device such that the drawbacks described above may be avoided.
It is an object of the present invention to provide new and improved device for conveying and guiding a lead-in strip of a web which does not take up much space.
It is another object of the present invention to provide a new and improved device for conveying and guiding a lead-in strip of a web which is readily serviceable.
It is a further object of the present invention to provide a new and improved device for conveying and guiding a lead-in strip of a web which does not require a large amount of air thereby allowing the amount of air used for producing a vacuum effect to be minimized.
It is another object of the present invention to provide a new and improved device for conveying and guiding a lead-in strip of a web which can be regulated in a longitudinal direction.
In order to achieve these objects and others, a device for conveying and guiding a lead-in strip according to the invention includes a conveyor formed of an air permeable material, the conveyor having a run with opposed faces on respective sides, the lead-in strip being conveyed on a first face on a first side of the run, and a foil rib located on a second side of the run of the conveyor, the foil rib comprising a foil head for producing a vacuum effect on an outlet side of the foil head in the direction of movement of the run.
More specifically, on the conveying run of the conveyor belt in accordance with invention, inside the loop of the run, foil ribs are fitted whose heads are in contact with or in the immediate vicinity of the conveyor belt or wire or equivalent, which foil ribs cause a vacuum level to be produced on the outlet face. In connection with the foil ribs, blow nozzles are provided for blowing in the direction of the foil such that a vacuum area is achieved over the distance between two foil heads. The foil head provides a vacuum area without an external source of air as the head guides air away from its outlet side.
In accordance with another embodiment of the invention, the conveyor belt/band/wire is rotated by an electric motor by means of a cogged belt or by a compressed air motor from the end of a roll. The advantages of the cogged belt drive include non-slipping acceleration and deceleration, an even driving speed and easy controllability. Air blown through the compressed-air motor or obtained from a separate compressed-air source is passed into foil ribs which are placed under the conveyor belt and by means of which a vacuum can be produced under the wire. The angle of the foil can be regulated, thereby allowing the vacuum level of the foil to be regulated. If a desired vacuum level is not achieved by the action of the foil ribs only, it is possible to utilize the Coanda effect which is provided by means of compressed air or from residual air of the compressed-air motor by blowing air through a nozzle fitted in connection with the foil rib along the face of the foil rib. The blow nozzle may be divided into two or more sectors in the cross direction in order to regulate the cross direction blow capacity.
In accordance with an additional feature of the invention, the foil ribs are provided with curved guide faces which further guide the air flow such that the vacuum over the entire length between the foil ribs will remain as desired, and a harmful pressure pulse of positive pressure will not be generated.
The arrangement accomplished by means of a compressed-air motor in accordance with the invention provides its vacuum by itself, and no external source of air is needed. Thus, the consumption of air can be minimized. Controllability is provided by regulating the angle of the foil or the amount of blown air. The distance between the foil ribs is chosen such that a desired vacuum effect can be maintained.
In accordance with one embodiment, a high vacuum is used in the first foil nozzle, and when the conveyor belt is above the web, a vacuum is also needed for other nozzles. In certain applications, subsequent nozzles are not always needed, for example, in applications in which the transfer distance is not long and the web is situated above the conveyor belt. The vacuum level is regulated by regulating the foil angle or the pressure or the amount of the air blown from the foil and, when needed, a blowing can be provided at the end of the conveyor belt loop before a reversing roll for the purpose of separating the lead-in strip from the conveyor belt.
The friction surface in the arrangement in accordance with the invention is almost nonexistent, thereby allowing relatively small motors to be used. Owing to low friction, the wear of the conveyor belt is also minimal, which increases the service life of the conveyor belt.
The arrangement in accordance with the invention may be accomplished such that a number of devices in accordance with the invention are placed one after the other forming a conveyor with a module construction for long draws.
The arrangement in accordance with the invention is of light construction and easy to service.
The invention is suitable for several different places of application in a paper machine, for example, for a press section, a size press, a coater, for a transfer from a dryer section to a calender or for a transfer from a calender to a reel-up. The invention may also be used when the web is passed over open nips, for example, when using the on-line arrangement marketed under the current assignee""s trademark OptiLoad, and for a transfer from a dryer section to a reel-up as well as in on-machine coating devices. As is clear from the examples listed above, the device in accordance with the invention is applicable to several different draws in open gaps of a paper machine.
The invention can be readily combined with various other threading devices, threading plates and threading blowings, etc. known in themselves.
In another embodiment of the device in accordance with invention, foil nozzles may also be arranged in the longitudinal direction of the device, in which connection a vacuum in the longitudinal direction can be produced.
In addition, the angle of the foil ribs in accordance with the invention with respect to the running direction of the web can be regulated from a cross direction to a longitudinal direction in order to achieve a desired effect and in order to affect the position of the lead-in strip on the conveyor wire in a lateral direction. The nozzles used may be slit or hole nozzles.
In the following, the invention will be described in more detail with reference to the figures in the accompanying drawing, to the details of which the invention is not by any means intended to be narrowly confined.